Lock-spindle blind fasteners, of the pull type, are generally of two types. One in which the fastener is constructed to be set by a double-action tool and the other constructed to be set by a single action tool. The fastener of this invention is of this latter type. Both type of tools are well known in the fastener art.
Fasteners constructed to be set by a single action tool have had a problem of so supporting a locking collar before and during the initial setting of the fastener that the collar will not cock or be improperly displaced causing binding against the mandrel and a malfunction in the final setting of the fastener.
Also, conventional countersinks in "as-prepared" holes are 100.degree. plus or minus 1.degree.. With prior conventional 100.degree. head rivets, also plus or minus 1.degree. tolerance, it is possible to have a 99.degree. fastener head in a 101.degree. installation hole, resulting in a loss of peripheral head contact with the countersink of the hole and creating potential sites for corrosion.
The under angle of the fastener head as related to the initial straight counterbore in the sleeve and to the entrant resistance angle for reception of the collar assure that there is full peripheral contact between the countersunk hole and the underside of the sleeve head and that the inter-lock between the sleeve and the mandrel is properly accomplished when set with a single action tool.